R1:79f57cf5171d

Split Herald rule creation across several steps

Summary:
Ref T4264. Currently, you choose a rule's content type (revision, commit, hook) and rule type (global, personal) on the same screen.

- I want to make some rule types unavailable for some content types (e.g., personal hooks make little sense).
- I want to make content type selection use a radio control instead of a dropdown, so it can explain what the content types do in more detail.
- For new "object" hooks, I want to add a third step where…